Why Wheel Aerodynamics Can Outweigh Wheel Weight and Inertia
Written by: Tom Anhalt
Date: Tue May 31 2011
Here's the example: I took an actual acceleration of mine from a race file. In this case it was an attack to go for a mid-race prime on a flat section of a crit course. In the span of 5s I accelerated from 25.5 mph to 30.4 mph with an average of 766W expended over those 5s (1s peak of 1080W). Although that's just one of my lowly Cat 4 examples, I think that it's representative of a fairly significant acceleration effort and would be a "worst case scenario" for any wheel inertia effects.
So, I took a look at what the peak forces at the pedal (assumed to be ~2X the average pedal force around the crank cycle) would need to be to create that acceleration due to the different "loads," i.e. merely accelerating the mass, overcoming the increase in aero drag, and also overcoming the increase in rolling resistance with velocity. I then took a look at what a difference in mass of 400g would mean to the situation. It's a somewhat simplistic look at it, but here's how it broke down:
- Increase in peak force (average over 5s) at the pedal just to accelerate the mass of myself plus the bike = 58.6 lbf. (this is the average ABOVE what it took to go steady state at 25.5 mph).
- Increase in peak force at the pedal to overcome increased aero drag at end of 5s span = 14.7 lbf.
- Increase in peak force at the pedal to overcome increased rolling resistance at end of 5s span = 11.5 lbf.
- Increase in peak force (average over 5s) at the pedal if mass above is increased by 400g = 0.3 lbf.
As can be seen in the pie chart to the left, the vast majority of the increase in peak force felt at the pedal was due to just linearly accelerating my (and my bike's) mass. Additionally, the increases in aero drag and rolling resistance accounted for approximately 1/3 of of the total increase in peak pedal force. The effect of increasing the rim mass of the wheels by nearly a full pound (400g) resulted in an increase in the average peak pedal force over the 5s acceleration of just 1%…in other words, an amount not likely to be "felt" at the pedals. It's pretty clear which areas of wheel performance the most benefit can be gained from making improvements: in the aerodynamics of the wheels and the rolling resistance of the tires and tubes applied to them.
During the development of the Firecrest 404 wheels, Zipp engineers decided to take a slightly different approach than has typically been used in thinking about and designing the rim shape. So, what's the bottom line? How well did the engineers do? 6.06.11
Zipp's 101 wheelset looks like a training wheelset, but they perform like they were meant for racing. So, which are these all-aluminum but still toroidal wheels? It seems the best answer is both. 4.21.12
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Acceleration and Aero contribution
Reviewed by: Dan Phillips, Jul 12 2011 10:05AM
What about handling?
Reviewed by: Eric, Jul 11 2011 4:11AM
Paul, I think you're missing the point...
Reviewed by: Tom Anhalt, Jun 22 2011 1:39PM
There is a difference and it can be significant - depending on the type of road and climb
Reviewed by: paul casino, Jun 22 2011 7:23AM
Wow...looks like there's some good responses here...
Reviewed by: Tom Anhalt, Jun 11 2011 7:29PM
- a roller test might be a good way of figuring out the "cost" of just the wheel inertia differences, except it leaves off the major source of inertia...the rider itself. The physics is pretty simple to calculate the forces required based on just the masses and geometry, so I'm not seeing how roller testing is going to tell us much. The high acceleration case I used in the analysis, along with the worst case assumption of ALL of the mass being added to the outer circumference of the wheels only increased the peak force at the pedal by less than 1 lbf.
- After reading the article, does anyone really think they'll be able to "feel" a difference in acceleration from moving 30g of nipple mass from the rims to the hubs? Sure, it'll change the rotational inertia of the wheels...but the fact remains that the rotational inertia of the wheels is an exceedingly small portion of the total inertia and forces on the bike+rider system.
- Although the speed was 25-30mph, that's not what's important...it's the acceleration that's important because that's what results in a force difference at the pedal. Whether it's from zero mph, or above 20 mph, the same rate of acceleration will cause the same peak force differences at the pedal.
- Lastly, I didn't say that mass doesn't matter...I'm only saying that when selecting wheels, wheel mass and inertia shouldn't be one of the first things you worry about.